The present invention relates to a composition of an electorlytic polishing solution used in carrying out the mirror polishing of niobium materials, above all to a method for reproducing the compostion which has been deteriorated in qualities.
The mirror polishing of the niobium (Nb) material is an essential technique to inhibit energy loss by preventing a high frequency absorption on the surface of the niobium material, when the niobium material is employed for accelerated cavity resonators in which its ultraconductivity is utilized. Such a mirror polishing of the niobium material has been carried out by an electrolytic polishing method similarly to the polishing treatment of other metallic materials. An electrolytic polishing solution used in such a conventional polishing treatment particularly comprises HF, H.sub.2 SO.sub.4 and water.
This electrolytic polishing technique for the niobium material is appreciably different from usual cases and will make progress in accordance with the mechanism that an oxide layer is formed on the surface of the niobium material by the action of H.sub.2 SO.sub.4 during electrical conduction and is then dissolved by the action of HF during interruption of the conduction.
With regard to the aforesaid conventional electrolytic polishing solution, however, the concentration of HF will noticeably drop due to the evaporation of HF and the consumption of HF in an electrolytic polishing reaction and the life of the solution will be extremely shortened. With the aim of overcoming such a problem, various researches have been conducted on methods for lengthening the life of the electrolytic polishing solution, and there are now known a method for supplementally adding hydrofluoric anhydride and another method for a replenishment of hydrofluoric acid.
However, hydrofluoric anhydride has a boiling point of 19.4.degree. C. and is thus gasified at ordinary temperature. Therefore, the material can be used only in a cool state of 0.degree. C. or less. In addition thereto, the hydrofluoric anhydride material is still liable to evaporate even in a cool state of 0.degree. C. or less and its vapor pressure will increase, which means that it is very dengerous at its handling. In the case of another method mentioned above, when hydrofluoric acid is supplementally added, the content of water in the electrolytic polishing solution will increase owing to the addition of water contained in hydrofluoric acid, with the result that a polishing performance will be deteriorated. Since having such problems, the methods of supplementally adding hydrofluoric anhydride and hydrofluoric acid are impractical in fact. Accordingly, in former days, the electrolytic polishing solution the concentration of HF in which has already dropped has been thrown away without being reused, which causes the increase in cost and which is a great obstacle to the improvement in a working efficiency at mass production.